It appears to me that your concept of the Flood is as if one were to take a
glass, put in some sand and gravel and fill it with water and shake the
whole mess for a while, then let it set and see what settles out. This may
indeed be the concept of some YECs also, however, the latest thinking in
Flood catastrophism involves a series of asteroid impacts which set up
mega-tsunami (impanami?) which sweep ashore with high erosion near the
impact site and wide deposition as the wave energy depletes. Thus the
depositions contain what was near the impact sites and do not represent
what was setteling out of a single large mass of homogenously mixed water
load.
Since flooding mega-tsunami could only occure by impacts in large bodies of
water, one would expect that the first depositions would contain primarily
marine life. As the mega-tsuanmi sweep inland (several waves would
generate from a single impact and each would follow the previous one ashore
before the first had time to drain back off) more and more land would be
innundated. Following impacts on continental areas now covered by
mega-tsunami distributed waters would start making depositions of land
based plants and animals. The more moble animals (and birds) would be
fleeing the mega-tunami for higher ground thus being (in general) the later
ones to begin to be deposited. Body boyancy would also play a part, but
not likely a major part. At this time, more detailed explanations that may
account for the apparent ordering are lacking. But time will tell.
Just how this model can explain the deposition of forams, nannoplankton and
diatoms as described, I don't know at this time. But, in my optimism for
the model, I expect an explanation can be found.
Allen